29                    Plants without Seeds: From Sea to Land
                Residents of the coal-producing central Chine...

   The earliest terrestrial plants invaded
the land sometime d...

29.1         Classification of Plantsa

 PHYLUM                                        ...
(a) Chara sp. (stonewort)                                                   PLANTS WITHOUT SEEDS: FROM SEA TO LAND        ...


576                             (a)
                                      Archegonia develop at the tip of a gametophyte. ...
The sporophytes of hornworts     Gametophytes are flat                PLANTS WITHOUT SEEDS: FROM SEA TO LAND              ...

travel. The hydroid may be the progenitor of the tracheid, the     ter they die. Rapidly g...

29.11 An Ancient Forest
This reconstruction is of a Carboniferous forest that once thrived in
what is now Michigan. The do...
PLANTS WITHOUT SEEDS: FROM SEA TO LAND                     581

   The presence of xylem indicated that these plants were ...
582     CHAPTER T WENT Y-NINE                                (a) Homospory                          The spores of homospor...

The Surviving Nonseed Tracheophytes


586     CHAPTER T WENT Y-NINE                                        Mature gametophyte (about 0.5 cm wide)


Introducing the Tracheophytes                               ...
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  1. 1. 29 Plants without Seeds: From Sea to Land Residents of the coal-producing central Chinese city of Changsha al- most never see the sun, because it is hidden behind an atmosphere dense with choking smog. Nine-tenths of the precipitation in Chang- sha is acid rain. China burns more coal than any other country in the world, and the resulting untreated smoke leads to disastrous conditions such as those in Changsha, the site of a major coal-fired power plant. Coal is used for 75 percent of China’s energy needs—primarily to generate elec- tricity, but also directly for heating, smelting of metals, and other purposes. The United States produces more than half of its electricity by burning coal, and indeed has the largest coal reserves in the world. Extractable coal reserves in the U.S. exceed An Ingredient of Coal-Based Smog When coal burns, it produces the fly ash the total amount of oil available for pumping in all other countries combined. Where shown in this artificially colored image. did all this coal come from? When too much coal burns where the Coal comes from the remains of seedless plants that grew in great forests hundreds smoke cannot blow away, the result is dis- of millions of years ago. (The two other “fossil astrous smog. fuels”—petroleum and natural gas—come from the remains of plankton that lived in an- cient oceans.) Plant parts from those forests sank in swamps that were later covered by soil. Over millions of years, as the buried plant ma- terial was subjected to intense pressure and el- evated temperatures, coal formed. At the time those ancient forests flourished, the plant world also included relatives of to- day’s mosses. These “mossy” ancestors were the first plant life on dry land. Today, mosses are among the most abundant plants on Earth, yet they seem at first glance to lack adaptations to life on land. Mosses have no advanced in- ternal “plumbing system” to move water and nutrients within their bodies, and their leafy photosynthetic organs are only one cell thick. They require liquid water in order to repro- duce, and indeed, seem at first glance to be highly dependent on external moisture. Mosses and their relatives do have effective adapta- tions for life in terrestrial environments, how- ever, as is obvious from their wide distribution. Most live in moist habitats, but a few mosses even live in deserts.
  2. 2. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 571 The earliest terrestrial plants invaded the land sometime during the Paleozoic “Brown plants” era (see Table 22.1). These plants were Stramenopiles tiny, but their metabolic activities Ancestral helped convert parent rock into soil that organism “Red plants” Red algae could support the needs of their succes- sors. Larger and larger plants evolved rapidly (in geological terms), and by the Chlorophytes Carboniferous period (354–290 mya) great forests were widespread. How- Charophytes “Green plants” ever, few of the trees in those forests were like those we know today. During the tens of millions Plantae of years since the Carboniferous, those early trees have been Embryophytes replaced by the modern trees whose adaptations and ap- pearance are familiar to us. 29.1 What Is a Plant? There are three ways to define a plant king- In this chapter, we will see how members of the plant dom, depending on which clade is chosen. In this book, we use the kingdom invaded the land and evolved. Our descriptions most restrictive definition: plants as embryophytes. Here, the two green algal clades are not considered plants. here will concentrate on those plants that lack seeds. The next chapter completes our survey of the plant kingdom by con- sidering the seed plants, which dominate the terrestrial scene today. There are ten surviving phyla of plants The surviving members of the kingdom Plantae fall naturally The Plant Kingdom into ten phyla (Table 29.1). All members of seven of those The kingdom Plantae is monophyletic—all plants descend phyla possess well-developed vascular systems that trans- from a single common ancestor and form a branch of the evo- port materials throughout the plant body. We call these seven lutionary tree of life. The shared derived trait, or synapo- phyla, collectively, the tracheophytes because they all pos- morphy, of the plant kingdom is development from embryos sess conducting cells called tracheids. The tracheophytes protected by tissues of the parent plant. For this reason, constitute a clade. plants are sometimes referred to as embryophytes. Plants re- The remaining three phyla (liverworts, hornworts, and tain the derived features that they share with green algae: the mosses), which lack tracheids, were once considered classes use of chlorophylls a and b and the use of starch as a photo- of a single larger phylum. In this book we use the term non- synthetic storage product. Both plants and green algae have tracheophytes to refer collectively to these three phyla. The cellulose in their cell walls. nontracheophytes are sometimes collectively called bryo- There are other ways to define “plant” and “plant king- phytes, but in this text we reserve that term for their most fa- dom” and still come out with a monophyletic group (clade). miliar members, the mosses. Collectively, the nontracheo- For example, combining plants as defined above with a group phytes are not a monophyletic group. They are the three of green algae called the charophytes results in a mono- basal clades of the plant kingdom. phyletic plant kingdom with several shared derived traits, in- cluding the retention of the egg in the parent body. The addi- tion of the chlorophytes (the remainder of the green algae) to Life cycles of plants feature alternation of generations the group just described gives another monophyletic group, A universal feature of the life cycles of plants is the alterna- with synapomorphies including the possession of chlorophyll tion of generations. Recall from Chapter 28 that alternation b, that can be called a plant kingdom. There are no hard-and- of generations has two hallmarks: fast criteria for defining a kingdom (or any other taxonomic The life cycle includes both multicellular diploid indi- rank), so these definitions of the plant kingdom are all valid. viduals and multicellular haploid individuals. In this book, we choose to use the first definition given Gametes are produced by mitosis, not by meiosis. above, in which the kingdom Plantae comprises only the em- Meiosis produces spores that develop into multicellular bryophytes (Figure 29.1). Some botanists refer to a group haploid individuals. consisting of the Plantae plus the green algae as the “green plant kingdom,” to the red algae as the “red plant kingdom,” If we begin looking at the plant life cycle at a single-cell and to the stramenopiles as the “brown plant kingdom.” stage—the diploid zygote—then the first phase of the cycle
  3. 3. 572 CHAPTER T WENT Y-NINE 29.1 Classification of Plantsa PHYLUM COMMON NAME CHARACTERISTICS Nontracheophytes Hepatophyta Liverworts No filamentous stage; gametophyte flat Anthocerophyta Hornworts Embedded archegonia; sporophyte grows basally Bryophyta Mosses Filamentous stage; sporophyte grows apically (from the tip) Tracheophytes Nonseed tracheophytes Lycophyta Club mosses Microphylls in spirals; sporangia in leaf axils Pteridophyta Ferns and allies Differentiation between main axis and side branches Seed plants Gymnosperms Cycadophyta Cycads Compound leaves; swimming sperm; seeds on modified leaves Ginkgophyta Ginkgo Deciduous; fan-shaped leaves; swimming sperm Gnetophyta Gnetophytes Vessels in vascular tissue; opposite, simple leaves Pinophyta Conifers Seeds in cones; needlelike or scalelike leaves Angiosperms Angiospermae Flowering plants Endosperm; carpels; much reduced gametophytes; seeds in fruit a No extinct groups are included in this classification. features the formation, by mitosis and cytokinesis, of a mul- tilization) results in the formation of a diploid cell—the zy- ticellular embryo and eventually the mature diploid plant gote—and the cycle repeats. (Figure 29.2). This multicellular, diploid plant is the sporo- The sporophyte generation extends from the zygote through phyte (“spore plant”). the adult, multicellular, diploid plant; the gametophyte gener- Cells contained in sporangia (singular, sporangium, ation extends from the spore through the adult, multicellular, “spore vessel”) on the sporophyte undergo meiosis to pro- haploid plant to the gamete. The transitions between the duce haploid, unicellular spores. By mitosis and cytokinesis, generations are accomplished by fertilization and meiosis. In a spore forms a haploid plant. This multicellular, haploid all plants, the sporophyte and gametophyte differ genetically: plant is the gametophyte (“gamete plant”) that produces The sporophyte has diploid cells, and the gametophyte has haploid gametes. The fusion of two gametes (syngamy, or fer- haploid cells. In the three basal plant clades, the gametophyte generation is larger and more self-sufficient, while the sporo- phyte generation is dominant in those groups that appeared Multicellular later in plant evolution. gametophyte Some protist life cycles also feature alternation of genera- Spore Gametes tions, suggesting that the plants arose from one of these pro- tist groups. But which one? HAPLOID (n) Meiosis Fertilization DIPLOID (2n) The Plantae arose from a green algal clade Much evidence indicates that the closest living relatives of Zygote the plants are members of a clade of green algae called the charophytes. The charophytes, along with some other green Multicellular algae and the plants, form a clade that is sister to the chloro- sporophyte phytes (see Figure 29.1), but we don’t yet know which charo- phyte clade is the true sister group to the plants. Stoneworts 29.2 Alternation of Generations A diploid sporophyte generation that produces spores alternates with a haploid gametophyte genera- of the genus Chara are charophytes that resemble plants in tion that produces gametes by mitosis. terms of their rRNA and DNA sequences, peroxisome con-
  4. 4. (a) Chara sp. (stonewort) PLANTS WITHOUT SEEDS: FROM SEA TO LAND 573 use different mechanisms for dispersing its gametes and progeny than its aquatic relatives, which can simply release them into the water. How did organisms descended from aquatic ancestors adapt to such a challenging environment? Adaptations to life on land distinguish plants from green algae Most of the characteristics that distinguish plants from green algae are evolutionary adaptations to life on land. Several of these features probably evolved in the common ancestor of the plants: The cuticle, a waxy covering that retards desiccation (drying) Gametangia, cases that enclose plant gametes and pre- vent them from drying out Embryos, which are young sporophytes contained within (b) Coleochaete sp. a protective structure 29.3 The Closest Relatives of Land Plants The plant kingdom Certain pigments that afford protection against the muta- probably evolved from a common ancestor shared with the charo- genic ultraviolet radiation that bathes the terrestrial phytes, a green algal group. (a) Molecular evidence seems to favor environment stoneworts of the genus Chara as sister group to the plants. (b) Thick spore walls containing a polymer that protects the Evidence from morphology indicates that the group including this coleochaete alga may be sister to the land plants. spores from desiccation and resists decay A mutualistic* association with a fungus that promotes nutrient uptake from the soil Further adaptations to the terrestrial environment appeared tents, mechanics of mitosis and cytokinesis, and chloroplast as plants continued to evolve. One of the most important of structure (Figure 29.3a). On the other hand, strong evidence these later adaptations was the appearance of vascular tissues. from morphology-based cladistic analysis suggests that the sister group of the plants is a group of charophytes that in- cludes the genus Coleochaete (Figure 29.3b). Coleochaete-like al- Most present-day plants have vascular tissues gae have several features found in plants, such as plasmod- The first plants were nonvascular, lacking both water-con- esmata and a tendency to protect the young sporophyte. ducting and food-conducting tissue. Although the term Whether they were more similar to stoneworts or to “nonvascular plants” is a time-honored name, it is mislead- Coleochaete, the ancestors of the plants lived at the margins of ing when applied to the entire nontracheophyte group, be- ponds or marshes, ringing them with a green mat. From cause some mosses (unlike liverworts and hornworts) do these marginal habitats, which were sometimes wet and have a limited amount of simple conducting tissue. Thus the sometimes dry, early plants made the transition onto land. more unwieldy name “nontracheophyte” is more descrip- tive. The first true tracheophytes—possessing specialized conducting cells called tracheids—arose later (Figure 29.4). The Conquest of the Land The nontracheophytes (the liverworts, hornworts, and Plants, or their immediate ancestors in the green mat, first in- mosses) have never been large plants. Except for some of the vaded the terrestrial environment between 400 and 500 mil- mosses, they have no water-conducting tissue, yet some are lion years ago. That environment differs dramatically from found in dry environments. Many grow in dense masses the aquatic environment. The most obvious difference is the (see Figure 29.9a), through which water can move by capil- availability of the water that is essential for life: It is every- lary action. Nontracheophytes also have leaflike structures where in the aquatic environment, but hard to find and to re- that readily catch and hold any water that splashes onto tain in the terrestrial environment. Water provides aquatic them. These plants are small enough that minerals can be dis- organisms with support against gravity; a plant on land, tributed throughout their bodies by diffusion. however, must either have some other support system or *In a mutualistic association, both partners—here, the plant and the fun- sprawl unsupported on the ground. A land plant must also gus—profit.
  5. 5. 574 CHAPTER T WENT Y-NINE Chlorophytes Ancestral alga Charophytes Liverworts Protected Hornworts Nontracheophytes embryos Mosses Club mosses Kingdom First true vascular Plantae Nonseed tissue tracheophytes Ferns and allies Tracheophytes First seed Gymnosperms plants Seed plants Flowering plants 29.4 From Green Algae to Plants Three key characteristics that emerged during plant evolution—protected embryos, vascular tis- sues, and seeds—are all adaptations to life in a terrestrial environ- We will examine the adaptations of the tracheophytes later ment. Plants with vascular tissue are called tracheophytes. in this chapter, concentrating first on the nontracheophytes. The Nontracheophytes: Familiar tracheophytes include the club mosses, ferns, Liverworts, Hornworts, and Mosses conifers, and angiosperms (flowering plants). Tracheophytes Most liverworts, horn- Liverworts differ from liverworts, hornworts, and mosses in crucial worts, and mosses grow Hornworts ways, one of which is the possession of a well-developed vas- in dense mats, usually Mosses cular system consisting of specialized tissues for the trans- in moist habitats. The Club mosses port of materials from one part of the plant to another. One largest of these plants are only Horsetails such tissue, the phloem, conducts the products of photosyn- about 1 meter tall, and most are Whisk ferns thesis from sites where they are produced or released to sites only a few centimeters tall or long. Ferns where they are used or stored. The other vascular tissue, the Why have the nontracheophytes not Gymnosperms xylem, conducts water and minerals from the soil to aerial evolved to be taller? The probable answer Flowering parts of the plant; because some of its cell walls are stiffened is that they lack an efficient system for con- plants by a substance called lignin, xylem also provides support in ducting water and minerals from the soil to dis- the terrestrial environment. tant parts of the plant body. To limit water loss, layers of ma- Nontracheophyte plants evolved tens of millions of years ternal tissue protect the embryos of all nontracheophytes. All before the earliest tracheophytes, even though tracheophytes nontracheophyte clades also have a cuticle, although it is of- appear earlier in the fossil record. The oldest tracheophyte ten very thin (or even absent in some species) and thus not fossils date back more than 410 million years, whereas the highly effective in retarding water loss. Nontracheophytes oldest nontracheophyte fossils are only about 350 million lack the leaves, stems, and roots that characterize tracheo- years old, dating from a time when tracheophytes were al- phytes, although they have structures analogous to each. ready widely distributed. This finding simply shows that, Most nontracheophytes live on the soil or on other plants, given the differences in their structures and the chemical but some grow on bare rock, dead and fallen tree trunks, and makeup of their cell walls, tracheophytes are more likely to even on buildings. Nontracheophytes are widely distributed form fossils than nontracheophytes are. over six continents and exist very locally on the coast of the
  6. 6. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 575 Archegonium (n) Gametophytes (n) Fertilization in nontracheophytes requires water so that sperm can H2O swim to eggs. Photosynthetic Egg (n) filament 29.5 A Nontracheophyte Life Cycle The life cycle of nontra- Bud Protonema cheophytes, illustrated here by a Antheridium (n) with bud moss, is dependent on an exter- Sperm (n) nal source of liquid water. The visible Rhizoid HAPLOID (n) green structure of nontracheophytes is Gametophyte generation the gametophyte. Germinating DIPLOID (2n) thus photosynthetic. Eventually ga- spore Sporophyte generation metes form within specialized sex Egg (n) organs, the gametangia. The arche- Ungerminated gonium is a multicellular, flask- spore shaped female sex organ with a Sporophyte (2n) long neck and a swollen base, which Meiosis Fertilization produces a single egg (Figure 29.6a). The antheridium is a male sex or- gan in which sperm, each bearing two flagella, are produced in large Capsule numbers (Figure 29.6b). Embryo (2n) Once released, the sperm must The sporophyte is attached to swim or be splashed by raindrops and nutritionally dependent to a nearby archegonium on the on the gametophyte. Gametophyte (n) same or a neighboring plant. The sperm are aided in this task by chemical attractants released by the egg or the archego- nium. Before sperm can enter the archegonium, certain cells seventh (Antarctica). They are successful plants, well adapted in the neck of the archegonium must break down, leaving a to their environments. Most are terrestrial. Some live in wet- water-filled canal through which the sperm swim to com- lands. Although a few nontracheophyte species live in fresh plete their journey. Note that all of these events require liq- water, these aquatic forms are descended from terrestrial uid water. ones. There are no marine nontracheophytes. On arrival at the egg, the nucleus of a sperm fuses with the egg nucleus to form a zygote. Mitotic divisions of the zy- gote produce a multicellular, diploid sporophyte embryo. Nontracheophyte sporophytes are dependent The base of the archegonium grows to protect the embryo on gametophytes during its early development. Eventually, the developing In nontracheophytes, the conspicuous green structure visible sporophyte elongates sufficiently to break out of the to the naked eye is the gametophyte (Figure 29.5). In contrast, archegonium, but it remains connected to the gametophyte the familiar forms of tracheophytes, such as ferns and seed by a “foot” that is embedded in the parent tissue and plants, are sporophytes. The gametophyte of nontracheo- absorbs water and nutrients from it. The sporophyte remains phytes is photosynthetic and therefore nutritionally inde- attached to the gametophyte throughout its life. The sporo- pendent, whereas the sporophyte may or may not be photo- phyte produces a capsule, within which meiotic divisions synthetic, but is always nutritionally dependent on the produce spores and thus the next gametophyte generation. gametophyte and remains permanently attached to it. The structure and pattern of elongation of the sporophyte A nontracheophyte sporophyte produces unicellular, hap- differ among the three nontracheophyte phyla—the liverworts loid spores as products of meiosis within a sporangium, or (Hepatophyta), hornworts (Anthocerophyta), and mosses capsule. A spore germinates, giving rise to a multicellular, (Bryophyta). The probable evolutionary relationships of these haploid gametophyte whose cells contain chloroplasts and are three phyla and the tracheophytes can be seen in Figure 29.4.
  7. 7. 576 (a) Archegonia develop at the tip of a gametophyte. (b) Antheridia are also located In the archegonium, the egg will be fertilized at the tip of a gametophyte. and begin development into a sporophyte. 29.6 Sex Organs in Plants (a) Archegonia and (b) anthe- ridia of the moss Mnium (phylum Bryophyta). The gametophytes of all plants have archegonia and antheridia, but they are much reduced in seed plants. These antheridia contain a large number of sperm. When released, the sperm can be carried by water The large egg cell is in the to an archegonium and then swim center of the archegonium. down its neck to the egg. Liverworts may be the most ancient surviving plant clade that shorten and compress a “spring” as they dry out. When The gametophytes of some liverworts (phylum Hepato- the stress becomes sufficient, the compressed spring snaps phyta) are green, leaflike layers that lie flat on the ground back to its resting position, throwing spores in all directions. (Figure 29.7a). The simplest liverwort gametophytes, however, Among the most familiar liverworts are species of the are flat plates of cells, a centimeter or so long, that produce genus Marchantia (Figure 29.7a). Marchantia is easily recog- antheridia or archegonia on their upper surfaces and anchor- nized by the characteristic structures on which its male and ing and water-absorbing filaments called rhizoids on their female gametophytes bear their antheridia and archegonia lower surfaces. Liverwort sporophytes are shorter than those (Figure 29.7b). Like most liverworts, Marchantia also repro- of mosses and hornworts, rarely exceeding a few millimeters. duces asexually by simple fragmentation of the gametophyte. The liverwort sporophyte has a stalk that connects capsule Marchantia and some other liverworts and mosses also repro- and foot. In most species, the stalk elongates and thus raises duce asexually by means of gemmae (singular, gemma), which the capsule above ground level, favoring dispersal of spores are lens-shaped clumps of cells. In a few liverworts, the gem- when they are released. The capsules of liverworts are simple: mae are loosely held in structures called gemmae cups, which a globular capsule wall surrounding a mass of spores. In some promote dispersal of the gemmae by raindrops (Figure 29.7c). species of liverworts, spores are not released by the sporophyte until the surrounding capsule wall rots. In other liverworts, however, the spores are thrown from the capsule by structures Hornworts evolved stomata as an adaptation to terrestrial life The phylum Anthocerophyta comprises the hornworts, so 29.7 Liverwort Structures Members of the phylum Hepatophyta named because their sporophytes look like little horns (Fig- display various characteristic structures. (a) Gametophytes. (b) Structures bearing antheridia and archegonia. (c) Gemmae cups. The umbrella-like structures The disc-headed These cups contain gemmae—small, lens- bear archegonia. structures bear antheridia. shaped outgrowths of the plant body, each capable of developing into a new plant. (a) Marchantia sp. (b) Marchantia sp. (c) Lunularia sp.
  8. 8. The sporophytes of hornworts Gametophytes are flat PLANTS WITHOUT SEEDS: FROM SEA TO LAND 577 can reach 20 cm in height. plates a few cells thick. sible interpretation of the current data. The exact evolutionary status of the hornworts is still unclear, and in some phyloge- netic analyses they are placed as the most ancient plant clade. Water and sugar transport mechanisms emerged in the mosses The most familiar nontracheophytes are the mosses (phylum Bryophyta). There are more species of mosses than of liver- worts and hornworts combined, and these hardy little plants are found in almost every terrestrial environment. They are often found on damp, cool ground, where they form thick Anthoceros sp. mats (Figure 29.9a). The mosses are probably sister to the tra- cheophytes (see Figure 29.4). 29.8 A Hornwort The sporophytes of hornworts can resemble little horns. Many mosses contain a type of cell called a hydroid, which dies and leaves a tiny channel through which water can ure 29.8). Hornworts appear at first glance to be liverworts with very simple gametophytes. These gametophytes consist of flat plates of cells a few cells thick. However, the hornworts, along with the mosses and tra- cheophytes, share an advance over the liverwort clade in their adaptation to life on land. They have stomata—pores that, when open, allow the uptake of CO2 for photosynthesis and the release of O2. Stomata may be a shared derived trait (synapomorphy) of hornworts and all other plants except liv- erworts, although hornwort stomata do not close and may have evolved independently. Hornworts have two characteristics that distinguish them from both liverworts and mosses. First, the cells of hornworts each contain a single large, platelike chloroplast, whereas the cells of other nontracheophytes contain numerous small, lens- shaped chloroplasts. Second, of all the nontracheophyte sporophytes, those of the hornworts come closest to being ca- pable of indeterminate growth (growth without a set limit). Liverwort and moss sporophytes have a stalk that stops growing as the capsule matures, so elongation of the sporo- phyte is strictly limited. The hornwort sporophyte, however, has no stalk. Instead, a basal region of the capsule remains ca- pable of indefinite cell division, continuously producing new spore-bearing tissue above. The sporophytes of some horn- worts growing in mild and continuously moist conditions can (a) become as tall as 20 centimeters. Eventually the sporophyte’s growth is limited by the lack of a transport system. To support their metabolism, the hornworts need access to nitrogen. Hornworts have internal cavities filled with mu- (b) The teeth of this moss capsule help cilage; these cavities are often populated by cyanobacteria expel the thousands of spores. that convert atmospheric nitrogen gas into a form usable by the host plant. 29.9 The Mosses (a) Dense moss forms hummocks in a valley on We have presented the hornworts as sister to the clade con- New Zealand’s South Island. (b) The moss capsule, from which spores sisting of mosses and tracheophytes, but this is only one pos- are dispersed, grows at the tip of the plant.
  9. 9. 578 CHAPTER T WENT Y-NINE travel. The hydroid may be the progenitor of the tracheid, the ter they die. Rapidly growing upper layers compress the characteristic water-conducting cell of the tracheophytes, but deeper-lying, decomposing layers. Partially decomposed it lacks lignin (a waterproofing substance that also lends plant matter is called peat. In some parts of the world, people structural support) and the cell wall structure found in tra- derive the majority of their fuel from peat bogs. Sphagnum- cheids. The possession of hydroids and of a limited system dominated peatlands cover an area approximately half as for transport of sucrose by some mosses (via cells called lep- large as the United States—more than 1 percent of Earth’s toids) shows that the old term “nonvascular plant” is some- surface. Long ago, continued compression of peat composed what misleading when applied to mosses. primarily of other nonseed plants gave rise to coal. In contrast to liverworts and hornworts, the sporophytes With their simple system of internal transport, the mosses of mosses and tracheophytes grow by apical cell division, in are, in a sense, vascular plants. However, they are not tra- which a region at the growing tip provides an organized pat- cheophytes because they lack true xylem and phloem. tern of cell division, elongation, and differentiation. This growth pattern allows extensive and sturdy vertical growth of sporophytes. Apical cell division is a shared derived trait Introducing the Tracheophytes of mosses and tracheophytes. Although they are an extraordinarily large and diverse The moss gametophyte that develops following spore ger- group, the tracheophytes can be said to have been launched mination is a branched, filamentous structure called a pro- by a single evolutionary event. Sometime during the Paleo- tonema (see Figure 29.5). Although the protonema looks a bit zoic era, probably well before the Silurian period (440 mya), like a filamentous green alga, it is unique to the mosses. Some the sporophyte generation of a now long-extinct plant pro- of the filaments contain chloroplasts and are photosynthetic; duced a new cell type, the tracheid (Figure 29.10). The tra- others, called rhizoids, are nonphotosynthetic and anchor the cheid is the principal water-conducting element of the xylem protonema to the substratum. After a period of linear growth, in all tracheophytes except the angiosperms, and even in the cells close to the tips of the photosynthetic filaments divide angiosperms, tracheids persist alongside a more specialized rapidly in three dimensions to form buds. The buds eventu- and efficient system of vessels and fibers derived from them. ally differentiate a distinct tip, or apex, and produce the fa- The evolution of a tissue composed of tracheids had two miliar leafy moss shoot with leaflike structures arranged spi- important consequences. First, it provided a pathway for rally. These leafy shoots produce antheridia or archegonia (see long-distance transport of water and mineral nutrients from Figure 29.6). The antheridia release sperm that travel through a source of supply to regions of need. Second, its stiff cell liquid water to the archegonia, where they fertilize the eggs. walls provided something almost completely lacking—and Sporophyte development in most mosses follows a pre- unnecessary—in the largely aquatic green algae: rigid struc- cise pattern, resulting ultimately in the formation of an ab- tural support. Support is important in a terrestrial environ- sorptive foot anchored to the gametophyte, a stalk, and, at ment because plants tend to grow upward as they compete the tip, a swollen capsule, the sporangium. In contrast to for sunlight to power photosynthesis. Thus the tracheid set hornworts, whose sporophytes grow from the base, the moss the stage for the complete and permanent invasion of land sporophyte stalk grows at its apical end, as tracheophytes do. by plants. Cells at the tip of the stalk divide, supporting elongation of The tracheophytes feature another evolutionary novelty: the structure and giving rise to the capsule. For a while, the a branching, independent sporophyte. A branching sporo- archegonial tissue grows rapidly as the stalk elongates, but phyte can produce more spores than an unbranched body, eventually the archegonium is outgrown and is torn apart by and it can develop in complex ways. The sporophyte of a tra- the expanding sporophyte. cheophyte is nutritionally independent of the gametophyte The lid of the capsule is shed after the completion of meio- at maturity. Among the tracheophytes, the sporophyte is the sis and spore development. In most mosses, groups of cells large and obvious plant that one normally notices in nature. just below the lid form a series of toothlike structures sur- This pattern is in contrast to the sporophyte of nontracheo- rounding the opening. Highly responsive to humidity, these phytes such as mosses, which is attached to, dependent on, structures dig into the mass of spores when the atmosphere is and usually much smaller than the gametophyte. dry; then, when the atmosphere becomes moist, they fling The present-day evolutionary descendants of the early tra- out, scooping out the spores as they go (Figure 29.9b). The cheophytes belong to seven distinct phyla (see Figure 29.10). spores are thus dispersed when the surrounding air is The tracheophytes have two types of life cycles, one that in- moist—that is, when conditions favor their subsequent ger- volves seeds and another that does not. The nonseed tra- mination. cheophytes (the two basal phyla) include the club mosses Mosses of the genus Sphagnum often grow in swampy and the ferns and their relatives: horsetails and whisk ferns. places, where the plants begin to decompose in the water af- We will describe these phyla in detail after taking a closer
  10. 10. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 579 Nontracheophytes Common Club mosses ancestor Horsetails Nonseed Tracheids; tracheophytes branching, Pteridophytes Whisk ferns independent sporophyte Tracheophytes Ferns Multiflagellate sperm, complex leaves Cycads Conifers Gymno- sperms Seed Seeds plants Ginkgos Gnetophytes Flowers, carpels, triploid endosperm Angiosperms 29.10 The Evolution of Today’s Plants The nine phyla of extant tracheophytes are divided between those that produce seeds and those that do not. ferns flourished in the tropical swamps of what would be- come North America and Europe (Figure 29.11). The remnants look at tracheophyte evolution. The five phyla of seed plants of those forests are with us today as huge deposits of coal. will be described in the following chapter. In the subsequent Permian period, the continents came to- gether to form a single gigantic land mass, called Pangaea. The continental interior became warmer and drier, but late Tracheophytes have been evolving in the period glaciation was extensive. The 200-million-year for almost half a billion years reign of the lycopod–fern forests came to an end as they were The evolution of an effective cuticle and of protective layers replaced by forests of seed plants (gymnosperms), which for the gametangia (archegonia and antheridia) helped make dominated until other seed plants (angiosperms) became the first tracheophytes successful, as did the initial absence dominant less than 80 million years ago. of herbivores (plant-eating animals) on land. By the late Sil- urian period, tracheophytes were being preserved as fossils that we can study today. Two groups of nonseed tracheo- The earliest tracheophytes lacked roots and leaves phytes that still exist made their first appearances during the The earliest known tracheophytes belonged to the now- Devonian period (409–354 mya): the lycopods (club mosses) extinct phylum Rhyniophyta. The rhyniophytes were among and the pteridophytes (including horsetails and ferns). Their the only tracheophytes in the Silurian period. The landscape proliferation made the terrestrial environment more hos- at that time probably consisted of bare ground, with stands pitable to animals. Amphibians and insects arrived soon af- of rhyniophytes in low-lying moist areas. Early versions of ter the plants became established. the structural features of all the other tracheophyte phyla ap- Trees of various kinds appeared in the Devonian period peared in the rhyniophytes of that time. These shared fea- and dominated the landscape of the Carboniferous. Mighty tures strengthen the case for the origin of all tracheophytes forests of lycopods up to 40 meters tall, horsetails, and tree from a common nontracheophyte ancestor.
  11. 11. 29.11 An Ancient Forest This reconstruction is of a Carboniferous forest that once thrived in what is now Michigan. The dominant “trees” are lycopods of the genus Lepidodendron; ferns are also abundant. In 1917, the British paleobotanists Robert Kidston and William H. Lang reported their finding of well-preserved fos- Sporangia sils of tracheophytes embedded in Devonian rocks near Rhynie, Scotland. The preservation of these plants was remarkable, con- 29.12 An Ancient Tracheophyte sidering that the rocks were more than 395 million years old. Relative This extinct plant, These fossil plants had a simple vascular system of phloem and Dichotomous Aglaophyton major (phylum xylem. Some of the plants had flattened scales on the stems, branching Rhyniophyta), lacked roots and leaves. It had a central column of which lacked vascular tissue and thus were not comparable to xylem running through its stems, the true leaves of any other tracheophytes. but true tracheids were lacking. These plants also lacked roots. They were apparently an- The rhizome is a horizontal chored in the soil by horizontal portions of stem, called rhi- underground stem, not a root. The aerial stems were less than zomes, that bore water-absorbing rhizoids. These rhizomes also 50 cm tall, and some were bore aerial branches, and sporangia—homologous with the topped by sporangia. Other very nontracheophyte capsule—were found at the tips of these similar rhyniophytes such as Rhynia did have tracheids. branches. Their branching pattern was dichotomous; that is, the shoot apex divided to produce two equivalent new branches, each pair diverging at approximately the same angle from the original stem (Figure 29.12). Scattered fragments of Rhizoids Rhizome such plants had been found earlier, but never in such profusion or so well preserved as those discovered by Kidston and Lang.
  12. 12. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 581 The presence of xylem indicated that these plants were tra- branched further. The underground portion could anchor the cheophytes. But were they sporophytes or gametophytes? plant firmly, and even in this primitive condition it could ab- Close inspection of thin sections of fossil sporangia revealed sorb water and minerals. The discovery of fossil plants from that the spores were in groups of four. In almost all living the Devonian period, all having horizontal stems (rhizomes) nonseed tracheophytes (with no evidence to the contrary with both underground and aerial branches, supported this from fossil forms), the four products of meiosis and cytoki- hypothesis. nesis remain attached to one another during their develop- Underground and aboveground branches, growing in ment into spores. The spores separate only when they are sharply different environments, were subjected to very dif- mature, and even after separation their walls reveal the ex- ferent selection pressures during the succeeding millions of act geometry of how they were attached. Therefore, a group years. Thus the two parts of the plant axis—the aboveground of four closely packed spores is found only immediately af- shoot system and the underground root system—diverged ter meiosis, and a plant that produces such a group must be in structure and evolved distinct internal and external a diploid sporophyte—and so the Rhynie fossils must have anatomies. In spite of these differences, scientists believe that been sporophytes. Gametophytes of the Rhyniophyta were the root and shoot systems of tracheophytes are homolo- also found; they, too, were branched, and depressions at the gous—that they were once part of the same organ. apices of the branches contained archegonia and antheridia. Although they were apparently ancestral to the other tra- THE ORIGIN OF TRUE LEAVES. Thus far we have used the term cheophyte phyla, the rhyniophytes themselves are long “leaf” rather loosely. We spoke of “leafy” mosses and com- gone. None of their fossils appear anywhere after the De- mented on the absence of “true leaves” in rhyniophytes. In vonian period. the strictest sense, a leaf is a flattened photosynthetic struc- ture emerging laterally from a main axis or stem and pos- sessing true vascular tissue. Using this precise definition as Early tracheophytes added new features we take a closer look at true leaves in the tracheophytes, we A new phylum of tracheophytes—the Lycophyta (club see that there are two different types of leaves, very likely of mosses)—also appeared in the Silurian period. Another—the different evolutionary origins. Pteridophyta (ferns and fern allies)—appeared during the The first leaf type, the microphyll, is usually small and Devonian period. These two groups arose from rhyniophyte- only rarely has more than a single vascular strand, at least in like ancestors. These new groups featured specializations not plants alive today. Plants in the phylum Lycophyta (club found in the rhyniophytes, including one or more of the fol- mosses), of which only a few genera survive, have such sim- lowing: true roots, true leaves, and a differentiation between ple leaves. The evolutionary origin of microphylls is thought two types of spores. by some biologists to be sterile sporangia (Figure 29.13a). The principal characteristic of this type of leaf is that its vascular THE ORIGIN OF ROOTS. The rhyniophytes had only rhizoids arising from a rhizome with which to gather water and 29.13 The Evolution of Leaves (a) Microphylls are thought to minerals. How, then, did subsequent groups of tracheo- have evolved from sterile sporangia. (b) The megaphylls of pterido- phytes come to have the complex roots we see today? phytes and seed plants may have arisen as photosynthetic tissue It is probable that roots had their evolutionary origins as developed between branch pairs that were “left behind” as dominant a branch, either of a rhizome or of the aboveground portion branches overtopped them. of a stem. That branch presumably penetrated the soil and 1 A branching stem system 2 Flat plates of photosynthetic became progressively tissue developed between reduced and flattened. branches. (a) (b) Overtopping Vascular Megaphyll tissue Sporangium Sporangia Microphyll Time Time A sporangium evolved into a simple leaf. 3 The end branches evolved into the veins of leaves.
  13. 13. 582 CHAPTER T WENT Y-NINE (a) Homospory The spores of homosporous plants produce a single type of gametophyte with both male and female reproductive organs. strand departs from the vascular system of the stem in such a way that the structure of the stem’s Gametophyte vascular system is scarcely disturbed. This was Homosporous (n) true even in the fossil lycopod trees of the Car- plants produce a single type of spore. Archegonium (å) boniferous period, many of which had leaves (n) Antheridium (ç) many centimeters long. (n) The other leaf type is found in ferns and seed Spore (n) plants. This larger, more complex leaf is called a Eggs (n) Sperm (n) megaphyll. The megaphyll is thought to have arisen from the flattening of a dichotomously HAPLOID (n) branching stem system and the development of Meiosis Fertilization overtopping (a pattern in which one branch differ- DIPLOID (2n) entiates from and grows beyond the others). This change was followed by the development of pho- Spore mother cell (2n) Zygote (2n) tosynthetic tissue between the members of over- topped groups of branches (Figure 29.13b). Mega- phylls may have evolved more than once, in different phyla of tracheophytes showing over- Sporangium (2n) Embryo (2n) topping of branches. Sporophyte (2n) HOMOSPORY AND HETEROSPORY. In the most (b) Heterospory ancient of the present-day tracheophytes, both Heterosporous plants The spores of heterosporous plants the gametophyte and the sporophyte are inde- produce male and female gametophytes. produce two types of pendent and usually photosynthetic. Spores pro- spores: a larger megaspore Megagametophyte (å) duced by the sporophytes are of a single type, and a smaller microspore. (n) and they develop into a single type of gameto- phyte that bears both female and male reproduc- Microgametophyte (ç) tive organs. The female organ is a multicellular (n) archegonium, typically containing a single egg. Megaspore (n) Eggs (n) The male organ is an antheridium, containing Microspore (n) Sperm (n) many sperm. Such plants, which bear a single type of spore, are said to be homosporous HAPLOID (n) (Figure 29.14a). Meiosis Fertilization A different system, with two distinct types of DIPLOID (2n) spores, evolved somewhat later. Plants of this type are said to be heterosporous (Figure 29.14b). One Spore mother Spore mother Zygote (2n) type of spore, the megaspore, develops into a cell (2n) cell (2n) larger, specifically female gametophyte (a megagametophyte) that produces only eggs. The Microsporangium other type, the microspore, develops into a Megasporangium (2n) Embryo (2n) (2n) smaller, male gametophyte (a microgametophyte) Sporophyte that produces only sperm. The sporophyte pro- (2n) duces megaspores in small numbers in megaspo- 29.14 Homospory and Heterospory (a) Homosporous plants bear a sin- rangia on the sporophyte, and microspores in gle type of spore. Each gametophyte has two types of sex organs, antheridia large numbers in microsporangia. (male) and archegonia (female). (b) Heterosporous plants, which bear two types of spores that develop into distinctly male and female gametophytes, The most ancient tracheophytes were all homo- evolved later. sporous, but heterospory evidently evolved inde- pendently several times in the early descendants of the rhyniophytes. The fact that heterospory evolved repeatedly suggests that it affords selective advan- Some tracheophyte clades arose and became extinct in the tages. Subsequent evolution in the plant kingdom featured course of evolution. The earliest clades to arise and survive ever greater specialization of the heterosporous condition. to this day belong to the nonseed tracheophytes.
  14. 14. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 583 The Surviving Nonseed Tracheophytes Strobilus The nonseed tracheophytes have a large, independent sporo- Microsporangium phyte and a small gametophyte that is independent of the sporophyte. The gametophytes of the surviving nonseed tra- cheophytes are rarely more than 1 or 2 centimeters long and are short-lived, whereas their sporophytes are often highly visible; the sporophyte of a tree fern, for example, may be 15 or 20 meters tall and may live for many years. The most prominent resting stage in the life cycle of a non- seed tracheophyte is the single-celled spore. This feature makes their life cycle similar to those of the fungi, the green algae, and the nontracheophytes, but not, as we will see in the next chapter, to that of the seed plants. Nonseed tracheo- phytes must have an aqueous environment for at least one stage of their life cycle because fertilization is accomplished by a motile, flagellated sperm. (a) Lycopodium obscurum (b) The ferns are the most abundant and diverse group of 29.15 Club Mosses (a) Strobili are visible at the tips of this club nonseed tracheophytes today, but the club mosses and horse- moss. Club mosses have microphylls arranged spirally on their stems. tails were once dominant elements of Earth’s vegetation. A (b) A thin section through a strobilus of a club moss, showing microsporangia. fourth group, the whisk ferns, contains only two genera. In this section we’ll look at the characteristics of these four groups and at some of the evolutionary advances that ap- peared in them. species of club mosses. Although only a minor element of present-day vegetation, the Lycophyta are one of two phyla that appear to have been the dominant vegetation during the The club mosses are sister to the other tracheophytes Carboniferous period. One type of coal (cannel coal) is The club mosses and Liverworts formed almost entirely from fossilized spores of the tree ly- their relatives (together Hornworts copod Lepidodendron—which gives us an idea of the abun- called lycopods, phy- Mosses dance of this genus in the forests of that time (see Figure lum Lycophyta) diverged Club mosses 29.11). The other major elements of Carboniferous vegetation earlier than all other living tra- Horsetails were horsetails and ferns. cheophytes—that is, the remaining Whisk ferns tracheophytes share an ancestor that Ferns was not ancestral to the Lycophyta. Horsetails, whisk ferns, and ferns Gymnosperms There are relatively few surviving species of constitute a clade Flowering club mosses. plants Once treated as distinct Liverworts The lycopods have roots that branch dichoto- phyla, the horsetails, Hornworts mously. The arrangement of vascular tissue in their stems is whisk ferns, and ferns Mosses simpler than in the other tracheophytes. They bear only mi- form a clade, the phylum Club mosses crophylls, and these simple leaves are arranged spirally on Pteridophyta (pteridophytes, Horsetails the stem. Growth in club mosses comes entirely from apical or “ferns and fern allies”). Within Whisk ferns cell division, and branching is dichotomous, by a division of that clade, the whisk ferns and the Ferns the apical cluster of dividing cells. horsetails are both monophyletic; the Gymnosperms The sporangia in many club mosses are contained within ferns are not. However, about 97 percent of Flowering conelike structures called strobili (singular, strobilus; Figure all fern species, including those with which plants 29.15). A strobilus is a cluster of spore-bearing leaves inserted you are most likely to be familiar, do belong to a on an axis tucked into the upper angle between a specialized single clade, the leptosporangiate ferns. In the pteridophytes— leaf and the stem. (Such an angle is called an axil.) Other club and in all seed plants—there is differentiation (overtopping) mosses lack strobili and bear their sporangia in the axil be- between the main axis and side branches. tween a photosynthetic leaf and the stem. This placement contrasts with the apical sporangia of the rhyniophytes. HORSETAILS GROW AT THE BASES OF STEM SEGMENTS. Like the There are both homosporous species and heterosporous club mosses, the horsetails are represented by only a few
  15. 15. 584 CHAPTER T WENT Y-NINE Leaves present-day species. All are in a single genus, Sporangium Equisetum. These plants are sometimes called Sporangiophore Fertile shoot “scouring rushes” because silica deposits found in their cell walls made them useful for cleaning. They have true roots that branch irregularly. Their sporangia curve back toward the stem on the ends of short stalks called sporangiophores (Figure 29.16a). Horsetails have a large sporophyte and a small gametophyte, both independent. The small leaves of horsetails are reduced megaphylls and form in distinct whorls (circles) around the stem (Figure 29.16b). Growth in horse- tails originates to a large extent from discs of di- viding cells just above each whorl of leaves, so each segment of the stem grows from its base. Such basal growth is uncommon in plants, al- though it is found in the grasses, a major group of (a) Equisetum arvense (b) Equisetum palustre flowering plants. 29.16 Horsetails (a) Sporangia and sporangiophores of a horsetail. (b) Vege- tative and fertile shoots of the marsh horsetail. Reduced megaphylls can be seen PRESENT-DAY WHISK FERNS RESEMBLE THE MOST ANCIENT in whorls on the stem of the vegetative shoot on the right; the fertile shoot on the TRACHEOPHYTES. There once was some disagree- left is ready to disperse its spores. ment about whether rhyniophytes are entirely extinct. The confusion arose because of the exis- tence today of two genera of rootless, spore-bearing plants, even these small leaves have more than one vascular strand, Psilotum and Tmesipteris, collectively called the whisk ferns. and are thus megaphylls. Psilotum nudum (Figure 29.17) has only minute scales The ferns constitute a group that first appeared during the instead of true leaves, but plants of the genus Tmesipteris Devonian period and today consists of about 12,000 species. have flattened photosynthetic organs—reduced mega- The ferns are not a monophyletic group, although, as already phylls—with well-developed vascular tissue. Are these two mentioned, 97 percent of the species—the leptosporangiate genera the living relics of the rhyniophytes, or do they have ferns—do constitute a monophyletic group. The leptospo- more recent origins? rangiate ferns differ from the other ferns in having sporan- Psilotum and Tmesipteris once were thought to be evolu- gia with walls only one cell thick, borne on a stalk. tionarily ancient descendants of anatomically simple ances- tors. That hypothesis was weakened by an enormous hole in the geological record between the rhyniophytes, which apparently became extinct more than 300 million years ago, and Psilotum and Tmesipteris, which are modern plants. DNA sequence data finally settled the question in favor of a more modern origin of the whisk ferns from fernlike ances- tors. These two genera are a clade of highly specialized plants that evolved fairly recently from anatomically more complex ancestors by loss of complex leaves and true roots. Whisk fern gametophytes live below the surface of the ground and lack chlorophyll. They depend upon fungal partners for their nutrition. Ferns evolved large, complex leaves Psilotum nudum The sporophytes of the ferns, like those of the seed plants, have true roots, stems, and leaves. Their leaves are typically 29.17 A Whisk Fern Psilotum nudum was once considered by some to be a surviving rhyniophyte and by others to be a fern. It is large and have branching vascular strands. Some species now included in the phylum Pteridophyta, and it is widespread in the have small leaves as a result of evolutionary reduction, but Tropics and Subtropics.
  16. 16. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 585 (b) (a) Adiantum pedatum (c) Marsilea mutica 29.18 Fern Fronds Take Many Forms (a) The fronds of Northern maidenhair fern form a pattern in this photograph. (b) The “fiddle- head” (developing frond) of a common forest fern; this structure will ravines or beneath trees in forests. The sporangia of ferns are unfurl and expand to give rise to a complex adult frond such as those found on the undersurfaces of the fronds, sometimes cover- in (a). (c) The tiny fronds of a water fern. ing the whole undersurface and sometimes only at the edges. In most species the sporangia are found in clusters called sori (singular, sorus) (Figure 29.19). Ferns are characterized by fronds (large leaves with com- plex vasculature; Figure 29.18a). During its development, the fern frond unfurls from a tightly coiled “fiddlehead” (Figure The sporophyte generation dominates 29.18b). Some fern leaves become climbing organs and may the fern life cycle grow to be as much as 30 meters long. Inside the sporangia, fern spore mother cells undergo meio- Because they require water for the transport of the male sis to form haploid spores. Once shed, the spores travel great gametes to the female gametes, most ferns inhabit shaded, distances and eventually germinate to form independent ga- moist woodlands and swamps. Tree ferns can reach heights metophytes. Old World climbing fern, Lygodium microphyllum, of 20 meters. Tree ferns are not as rigid as woody plants, and is currently spreading disastrously through the Florida Ever- they have poorly developed root systems. Thus they do not glades, choking off the growth of other plants. This rapid grow in sites exposed directly to strong winds, but rather in spread is testimony to the effectiveness of windborne spores. Fern gametophytes have the potential to produce both an- theridia and archegonia, although not necessarily at the same time or on the same gametophyte. Sperm swim through wa- ter to archegonia—often to those on other gametophytes— where they unite with an egg. The resulting zygote develops into a new sporophyte embryo. The young sporophyte sprouts a root and can thus grow independently of the ga- metophyte. In the alternating generations of a fern, the ga- metophyte is small, delicate, and short-lived, but the sporo- phyte can be very large and can sometimes survive for hundreds of years (Figure 29.20). Most ferns are homosporous. However, two groups of aquatic ferns, the Marsileaceae and Salviniaceae, are derived from a common ancestor that evolved heterospory. The megaspores and microspores of these plants (which germi- Dryopteris intermedia nate to produce female and male gametophytes, respectively) are produced in different sporangia (megasporangia and mi- 29.19 Fern Sori Are Clusters of Sporangia Sori, each containing many spore-producing sporangia, have formed on the underside of crosporangia), and the microspores are always much smaller this frond of the Midwestern fancy fern. and greater in number than the megaspores.
  17. 17. 586 CHAPTER T WENT Y-NINE Mature gametophyte (about 0.5 cm wide) 29.20 The Life Cycle of a Fern The most conspicuous stage in the fern life cycle is the mature, diploid sporophyte. Egg Rhizoids Archegonium A few genera of ferns pro- duce a tuberous, fleshy gameto- phyte instead of the charac- Germinating spore teristic flattened, photosynthetic Sperm Antheridium structure produced by most ferns. Like the gametophytes of Spore tetrad whisk ferns, these tuberous gametophytes depend on a HAPLOID (n) mutualistic fungus for nutri- Meiosis Fertilization tion; in some genera, even the DIPLOID (2n) Mature sporophyte sporophyte embryo must be- (typically 0.3–1 m tall) come associated with the fun- gus before extensive develop- Archegonial wall ment can proceed. In Chapter 31 we will see that there are Embryo many other important plant– Sporophyte fungus mutualisms. Sporangium All the tracheophytes we have discussed thus far dis- perse themselves by spores. In the next chap- Sori (clusters of sporangia) Root ter we will discuss the plants that dominate most of Earth’s vegetation today, the seed Horizontal stem plants, whose seeds afford new sporophytes Roots protection unavailable to those of the nonseed tracheophytes. Chapter Summary Tracheophytes are characterized by possession of a vascular system, consisting of water- and mineral-conducting xylem and The Plant Kingdom nutrient-conducting phloem. Nontracheophytes lack a vascular Plants are photosynthetic eukaryotes that develop from system. Review Figure 29.4 embryos protected by parental tissue. Like the green algae, they use chlorophylls a and b and store carbohydrates as starch. The Nontracheophytes: Liverworts, Hornworts, and Review Figure 29.1 Mosses Plant life cycles feature alternation of gametophyte (haploid) Nontracheophytes either lack vascular tissues completely or, and sporophyte (diploid) generations. Both generations include in the case of certain mosses, have only a rudimentary system of multicellular organisms. Review Figure 29.2 water- and food-conducting cells. There are ten surviving phyla of plants. The three basal phyla The nontracheophyte sporophyte generation is smaller than are nontracheophytes, and the remaining seven phyla are tra- the gametophyte generation and depends on the gametophyte cheophytes. Review Table 29.1 for water and nutrition. Review Figures 29.5, 29.6. See Web/CD Tutorial 29.1 Plants arose from a common green algal ancestor in the charophyte clade, either a stonewort or a member of the group The nontracheophytes include the liverworts (phylum that includes Coleochaete. Descendants of this ancestral charo- Hepatophyta), hornworts (phylum Anthocerophyta), and moss- phyte colonized the land. es (phylum Bryophyta). Hornwort sporophytes grow at their basal end. The Conquest of the Land Hornworts, mosses, and tracheophytes have surface pores The acquisition of a cuticle, gametangia, a protected embryo, (stomata) that allow gas exchange and minimize water loss. protective pigments, thick spore walls with a protective poly- In mosses and tracheophytes, the sporophytes grow by apical mer, and a mutualistic association with a fungus are all defining cell division. characters of plants, and all are associated with the adaptation The hydroids of mosses, through which water may travel, of plants to life on land. may be ancestral to tracheids, the water-conducting cells of the tracheophytes.
  18. 18. PLANTS WITHOUT SEEDS: FROM SEA TO LAND 587 Introducing the Tracheophytes c. possess xylem and phloem. The tracheophytes have vascular tissue with tracheids and d. possess true leaves. other specialized cells designed to conduct water, minerals, and e. possess true roots. products of photosynthesis. 5. Which statement is not true of the mosses? Present-day tracheophytes are grouped into seven phyla. The a. The sporophyte is dependent on the gametophyte. two basal phyla are nonseed tracheophytes, and the rest are b. Sperm are produced in archegonia. seed plants. Review Figure 29.10 c. There are more species of mosses than of liverworts and In tracheophytes, the sporophyte is larger than the gameto- hornworts combined. phyte and independent of the gametophyte generation. d. The sporophyte grows by apical cell division. e. Mosses are probably sister to the tracheophytes. The earliest tracheophytes, known to us only in fossil form, lacked roots and leaves. Review Figure 29.12 6. Megaphylls Roots may have evolved from rhizomes or from branches a. probably evolved only once. that penetrated the ground. Microphylls are thought to have b. are found in all the tracheophyte phyla. evolved from sporangia, and megaphylls may have resulted c. probably arose from sterile sporangia. from the flattening and reduction of an overtopping, branching d. are the characteristic leaves of club mosses. stem system. Review Figure 29.13 e. are the characteristic leaves of horsetails and ferns. Heterospory, the production of distinct female megaspores 7. The rhyniophytes and male microspores, evolved on several occasions from a. possessed vessel elements. homosporous ancestors. Review Figure 29.14. See Web/CD b. possessed true roots. Activities 29.1 and 29.2 c. possessed sporangia at the tips of stems. d. possessed leaves. The Surviving Nonseed Tracheophytes e. lacked branching stems. Club mosses (phylum Lycophyta) have microphylls arranged 8. Club mosses and horsetails spirally. a. have larger gametophytes than sporophytes. Among the pteridophytes (phylum Pteridophyta), horsetails b. possess small leaves. have reduced megaphylls in whorls. Whisk ferns lack roots; one c. are represented today primarily by trees. genus has minute scales rather than leaves, and the other has d. have never been a dominant part of the vegetation. reduced megaphylls with vascular tissue. Leaves with more e. produce fruits. complex vasculature are characteristic of all other phyla of tra- 9. Which statement about ferns is not true? cheophytes. a. The sporophyte is larger than the gametophyte. The ferns are not a clade, although 97 percent of fern species b. Most are heterosporous. do constitute a clade. Ferns have megaphylls with branching vas- c. The young sporophyte can grow independently of the cular strands. Review Figure 29.20. See Web/CD Activity 29.3 gametophyte. d. The frond is a megaphyll. e. The gametophytes produce archegonia and antheridia. Self-Quiz 10. The leptosporangiate ferns 1. Plants differ from photosynthetic protists in that only plants a. are not a monophyletic group. a. are photosynthetic. b. have sporangia with walls more than one cell thick. b. are multicellular. c. constitute a minority of all ferns. c. possess chloroplasts. d. are pteridophytes. d. have multicellular embryos protected by the parent. e. produce seeds. e. are eukaryotic. 2. Which statement about alternation of generations in plants For Discussion is not true? a. It is heteromorphic. 1. Mosses and ferns share a common trait that makes water b. Meiosis occurs in sporangia. droplets a necessity for sexual reproduction. What is that trait? c. Gametes are always produced by meiosis. 2. Are the mosses well adapted to terrestrial life? Justify your d. The zygote is the first cell of the sporophyte generation. answer. e. The gametophyte and sporophyte differ genetically. 3. Ferns display a dominant sporophyte generation (with large 3. Which statement is not evidence for the origin of plants fronds). Describe the major advance in anatomy that enables from the green algae? most ferns to grow much larger than mosses. a. Some green algae have multicellular sporophytes and multicellular gametophytes. 4. What features distinguish club mosses from horsetails? What b. Both plants and green algae have cellulose in their cell walls. features distinguish these groups from rhyniophytes? From c. The two groups have the same photosynthetic and ferns? accessory pigments. 5. Why did some botanists once believe that the whisk ferns d. Both plants and green algae produce starch as their should be classified together with the rhyniophytes? principal storage carbohydrate. e. All green algae produce large, stationary eggs. 6. Contrast microphylls with megaphylls in terms of structure, evolutionary origin, and occurrence among plants. 4. The nontracheophytes a. lack a sporophyte generation. b. grow in dense masses, allowing capillary movement of water.